1. Field of the Invention
This invention relates to acid anhydrides and more particularly relates to those acid anhydrides which are suitable for use as an epoxy hardener or curing agent for resins.
2. History of the Prior Art
Anhydrides find many commercial uses. One of the major uses for anhydrides, especially certain anhydrides, is as curing agents or hardeners for epoxy resins. Many of the curing agents or hardeners for epoxy resins are acid anhydrides. Among the most widely used anhydrides for curing agents are methyltetrahydrophthalic acid anhydride (MeTHPAA). MeTHPAA is a relatively inexpensive anhydride formed by reaction of maleic anhydride with isoprene and is a liquid when isomerized to the one isomer, i.e., the isomer wherein the anhydride carbons and the double bond are attached to the same carbon atoms on the ring. MeTHPAA, however, has some disadvantages. In particular, the heat deflection temperature and impact strength of epoxy products cured with MeTHPAA are not as high as desireable. Of the low cost liquid anhydrides, MeTHPAA is the only anhydride having a viscosity below 100 cps.
Methylhexahydrophthalic acid anhydride (MeHHPAA) is another liquid anhydride which has a low viscosity but is a high cost anhydride which sometimes has stability problems. Hexahydrophthalic acid anhydride (HHPAA) is sometimes used instead of MeTHPAA as an epoxy curing agent. Unfortunately, HHPAA is a solid instead of a liquid at room temperature thus requiring that the HHPAA be melted and used at elevated temperatures. Interestingly, HHPAA can also be prepared as a low viscosity liquid product when an expensive palladium catalyst is used. Unfortunately, the liquid isomer reverts to a solid isomer upon heating. Furthermore, the HHPAA anhydride picks up water upon exposure to air. For these reasons, a commercial liquid HHPAA has been unacceptable and impractical. Solid HHPAA, nevertheless, continues to be used because it imparts good properties to cured epoxy resin, e.g., good color, good electrical characteristics, good strength and good heat resistance, even though liquid anhydrides such as MeTHPAA, methyl endomethylenetetrahydrophthalic anhydride (MEA), dodecenylsuccinic acid anhydride (DDSAA) and methylhexahydrophthalic acid anhydride (MeHHPAA) are available. Methyl endomethylenetetrahydrophthalic anhydride, also known as methyl-5-norbornene-2,3-dicarboxylic anhydride is available from Buffalo Color Corporation under the trademark Nadic.RTM. Methyl Anhydride. The impact strength of epoxy resin cured with HHPAA is not, however, as high as desired. MEA has a viscosity at 25.degree. C. of about 200 cps and DDSAA has a viscosity at 25.degree. C. of about 290 cps.